In recent years, in a flat panel display device such as a mobile phone or a personal digital assistant (PDA), or a flat pane, in order to enhance protection and beauty of a display, a thin sheet-shaped cover glass is arranged on a front surface of a display so as to cover a region wider than an image display area.
Weight reduction and thickness reduction are required for such a flat panel display device, and to achieve the requirement, a cover glass used for protecting a display is also required to reduce its thickness.
However, when the thickness of a cover glass is reduced, strength thereof is decreased, and the cover glass itself may break during use or by drop during carrying. Therefore, there is a problem that the primary role of protecting a display device cannot be performed.
For this reason, to improve scratch resistance, in the conventional cover glass, a float glass produced by a float process is chemically strengthened to form a compressive stress layer on the surface thereof, thereby enhancing scratch resistance of the cover glass.
In recent years, the higher scratch resistance is required for a cover glass and the like. The surface compressive stress of a chemically strengthened float glass obtained by chemically strengthening the conventional soda lime glass was about 500 MPa, and a depth of a compressive stress layer was approximately about 10 μm. To respond to the requirement of high scratch resistance, a chemically strengthened float glass having a surface compressive stress of 600 MPa or more and a depth of a compressive stress layer of 15 μm or more is developed.
It is reported that warpage occurs in a float glass after chemical strengthening, thereby deteriorating flatness (Patent Document 1). The warpage occurs by the difference of the degree of behavior of chemical strengthening between a glass surface that does not contact with molten tin during float forming (hereinafter referred to as a “top surface”) and a glass surface that contacts with molten tin during float forming (hereinafter referred to as a “bottom surface”).
The warpage of a float glass becomes large with increasing the degree of behavior of chemical strengthening. Therefore, in a chemically strengthened float glass having the surface compressive stress of 600 MPa or more and a depth of a compressive stress layer of 15 μm or more, which has been developed to respond to the requirement of high scratch resistance, the problem of warpage becomes more obvious as compared with the conventional chemically strengthened float glass having the surface compressive stress of about 500 MPa and a depth of a compressive stress layer of about 10 μm.
Conventionally, it has been considered that the reason that the degree of behavior of chemical strengthening differs between the top surface and the bottom surface in a float glass is due to that a molten metal invades the glass surface contacting the molten metal during float forming (Patent Document 1).
Patent Document 1 discloses that a sheet-shaped body produced by a float process and processed is chemically strengthened after dipping in or contacting Li ion, Na ion or a mixed inorganic salt thereof without surface polishing, thereby improving the warpage.
Furthermore, conventionally, to reduce the warpage of the float glass, a coping method of decreasing strengthening stress by chemical strengthening or removing a surface heterogeneous layer by subjecting a top surface and bottom surface of a float glass to grinding treatment or polishing treatment, and then chemically strengthening the float glass, has been carried out.